Electrostatic discharge (ESD) is a well-known problem. ESD is a fast discharge of current that can damage semiconductor chips or devices. The degree of damage to a device by an ESD event is determined by the device's ability to dissipate the energy of the discharge or ability to withstand the current levels involved. Some devices may be more readily damaged by discharges occurring within automated equipment, while others may be more prone to damage from handling by personnel.
If ESD damage renders a device completely nonfunctional, the device is simply discarded. However, ESD damage may be mild enough such that the ESD damage results in intermittent failures. Intermittent failures are typically more problematic, because such damage may not become apparent until the device is already in the field. Also, the performance of a damaged device may be compromised before the device actually malfunctions. Hence, it is important to identify devices that are sensitive or susceptible to ESD and to determine their level of sensitivity.
Well-known test procedures, which are used to characterize, determine, and classify the sensitivity of components to ESD, are based on the three main models of ESD events: human body model (HBM), machine model (MM), and charged device model (CDM). HBM is one of the most common causes of ESD damage. HBM simulates a direct transfer of electrostatic charge through a significant series resistor from a human body or from a charged material to the device. For example, when one walks across a floor, an electrostatic charge accumulates on the body. Simple contact of a finger to the leads of a device allows the body to discharge, possibly causing damage to the device.
MM simulates a discharge similar to an HBM event but the electrostatic charge originates from a charged conductive object, such as a metallic tool or fixture. For example, a rapid discharge may originate from a charged board assembly or from the charged cables of an automatic tester.
CDM simulates a discharge from a charged device. For example, a device may become charged from sliding down the feeder in an automated assembler. The charge from the device may then get rapidly transferred to another conductor, during which the device may be damaged. While the duration of the discharge is very short (e.g. less than one nanosecond), the peak current can reach several tens of amperes.
FIG. 1 is a block diagram of a conventional device under test (DUT) 50. The DUT 50 is coupled to a current source 52. The different types of ESD events require specific current levels, or ranges of current levels, for optimal ESD simulation and testing. For example, CDM ESD testing requires that the current source 52 be capable of sourcing a high current (e.g. 5 A) and sourcing the high current quickly (e.g. 1 ns).
FIG. 2 is a block diagram of the conventional DUT 50 of FIG. 1 coupled to a conventional ESD tester 60. The ESD tester 60 includes a voltage source 62, a transmission line 64, and a switch 66. In operation, the voltage source 62 provides charge to the transmission line 64, which uses storage of resonant energy to generate a high current. When the switch 66 turns on, an ESD current (i.e. an electrostatic discharge) is transferred from the transmission line 64 to the DUT 50.
The combination of the voltage source 62, the transmission line 64, and the switch 66 may also be referred to as a transmission line pulser.
A problem with conventional ESD testers is that they require specialized test equipment comprising a high-voltage power source and resonant energy storage elements. Conventional ESD testers are also very expensive (e.g. $200 K to $1 M, or more).
Furthermore, comprehensive ESD testing of all the pins individually on a chip is impractical, because the ESD test and functional test require separate and specialized equipment. For example, a DUT first undergoes ESD testing on an ESD tester. If the DUT passes ESD testing, the DUT is then functionally tested on a functional tester. As a result, the testing conditions do not correlate well with actual (i.e. non-simulated) ESD events.
Accordingly, what is needed is an improved system and method for ESD testing. The present invention addresses such a need.